1. Field of the invention
The present invention relates to the field of distributed feedback semiconductor lasers. More specifically, the invention makes it possible to develop single-mode distributed feedback lasers with a production rate close to 100% using a simple and robust technology.
2. Related Art
It is known that the first objective of distributed feedback is to eliminate as much as possible the secondary modes of the laser. A second objective is generally to reduce the mirror losses, that is, the losses due to the finite length of the laser cavity.
Currently, there are two main methods of producing distributed feedback (DFB) lasers.
These two known methods consist either in producing a modulation of the effective index of the active area, or in producing a modulation of the losses, also qualified as gain modulation. From a technological point of view, these methods generally rely on the etching of a network which can be buried, that is, etched after the growth of the active area but before the growth of the top waveguide of the laser, or located on the surface of the top waveguide of the laser and metallized.
The modulation of the index is theoretically preferred because it makes it possible to produce a DFB laser without increasing the losses, unlike loss modulation, normally easier to implement technologically, but the main defect of which is that it degrades the laser threshold.
Consequently, and since the common aim is to optimize the laser threshold, preference is given to a modulation of the index. However, while this method of producing DFB lasers makes it possible to eliminate a major proportion of the secondary modes, there generally remain two main modes, even in the case of a strongly coupled feedback.
Thus, to obtain a single-mode laser, other technologies are often added to that used to obtain a distributed feedback. The principle is to introduce significant additional losses on the spurious mode or modes.
The first method for obtaining a single-mode DFB laser consists in creating a feedback dominated by the optical losses. These losses are due to the absorption of the electromagnetic radiation by the constituent materials of the bottom and top waveguides of the laser. This modulation of the losses can only make it possible to have a single mode but, for the second, or side, mode rejection rates (SMSR) to be high enough, the losses introduced degrade the threshold of the laser. Furthermore, the amplitude of the optical losses that can be used to modulate the losses of the laser is very difficult to control. Finally, the optical losses systematically affect a wide area and therefore affect both DFB modes; thus, the mode that is to be prioritized is nevertheless affected.
A second technology involves introducing a defect in the etched network (buried network or metallized surface network). This defect makes it possible to favour the emergence of a predominant single mode in the middle of the prohibited band which would exist in the case of a defect-free periodic structuring. The main drawback of this technology is that it requires a large proportion of the geometrical parameters of the laser (notably length of cavities, position of mirrors, depth of networks) to be set first, because it breaks the translation invariance. Developing a laser using this technology is therefore restrictive and complex, all the more so as it is very sensitive to the accuracy of the technological implementation, and in particular the depth of the etched network.
In the prior art, there are yet other technologies with which to obtain single-mode lasers, such as the use of variable-pitch networks or the use of the distributed Bragg reflector (DBR) technology. However, these technologies, like the second technology described above, systematically impose strong geometrical constraints, making the production of lasers using these technologies more complex and sensitive.
To sum up, the main drawback of the main technologies of the prior art leading to the developing of single-mode DFB lasers lies either in the geometrical constraints that they impose, or in the degradation of the laser threshold. The faults mentioned make it difficult to control the technologies for producing single-mode DFB lasers and the production rates of such lasers according to these technologies are reduced.